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Space NASA Science

Mass of Dwarf Planet Eris 27% Greater than Pluto 220

jcgam69 writes "When it was discovered in 2005, some thought Eris should be considered the 10th planet of our solar system. Everyone still considered Pluto a planet then. At first, Eris was thought to be slightly larger. Now — with the help of Eris' moon — Eris is known to be 27% more massive than Pluto. If Pluto had remained a planet to the entire community of astronomers, surely Eris would be considered the 10th planet."
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Mass of Dwarf Planet Eris 27% Greater than Pluto

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  • by jd142 ( 129673 ) on Thursday June 14, 2007 @06:18PM (#19512817) Homepage
    That Mondas is the 10th planet. Duh.
  • It was a nickname (Score:4, Informative)

    by Kelson ( 129150 ) * on Thursday June 14, 2007 @06:21PM (#19512863) Homepage Journal

    (BTW, what ever happened to naming it Xena?)

    Xena was never more than an unofficial nickname. No one, including the person who discovered it, ever intended for Xena and Gabrielle to be the official names for this pair of heavenly bodies.

  • Re:If it's round (Score:2, Informative)

    by poopdeville ( 841677 ) on Thursday June 14, 2007 @06:29PM (#19512955)
    You're a member of the IAU? Otherwise, you must be using the Royal we.

    In any event, the "dwarf planet" classification is informative. Dwarf planets have sufficiently low mass that they have no managed to clear their orbital path from other massive objects. Their properties are very different, despite orbiting the sun and being round.

    Why use a cluttered ontology when a clean one can be designed?
  • Re:Silly question (Score:3, Informative)

    by Bramantip ( 1054582 ) <(lp.gro.xsuip) (ta) (sniknej)> on Thursday June 14, 2007 @06:33PM (#19512991) Homepage
    Gravity provides the centripetal force needed to keep satellites in orbit. If you focus on the simple case of circular orbits, you can use the centripetal force formula with the law of gravity to determine the mass of a planet. Simply set the force of gravity equal to the centripetal force and solve for the mass of the planet M.

    M = r * v^2 / G

    The period of Eris' moon provides another way to calculate its mass.

    Period T = 2pi * sqrt(r^3 / G*M)

    Thus I imagine the various images of the moon provided a way to calculate its period and indirectly determine the mass of the central body.

    However the article doesn't give any specifics. It would be interesting to know what methods they used and the degree of accuracy of their measurements.

    JJ +
  • Re:grandfathered (Score:3, Informative)

    by Metasquares ( 555685 ) <> on Thursday June 14, 2007 @06:35PM (#19513001) Homepage
    This is science we're talking about. 'Grandfathering' a planet would be like declaring 1 a prime because it was treated as one in the past. If we did this, we'd have to start explicitly making exclusions for the grandfathered planet/number ("all planets except Pluto are...").

    Consistency is important in science.
  • Everyone? (Score:2, Informative)

    by KenAndCorey ( 581410 ) on Thursday June 14, 2007 @06:47PM (#19513137)

    Everyone still considered Pluto a planet then [in 2005]

    What are you talking about? I haven't considered it a planet since I took astronomy in the early 90's. Of course the public didn't have a clue, but a lot of astronomers knew Pluto shouldn't be considered a (regular) planet.

  • Re:grandfathered (Score:3, Informative)

    by ZombieWomble ( 893157 ) on Thursday June 14, 2007 @06:49PM (#19513157)
    Except this isn't really science. This is a labeling problem. The only things this definition affects are other labeling issues, and occasional problems of ambiguity in certain forms of casual conversation. I can think of no real technical situation where this would cause an issue, and the whole fuss is rather silly and gives some people a fairly low opinion of science.

    As an example I was watching some random evangelical show late one night (I had insomnia) while the main debate was going on. There was a whole segment dedicated to how scientists "didn't know" whether Pluto was a planet or not and how this clearly meant that they couldn't possibly make definitive statements on things like evolution and so forth. Of course to anyone involved in science the flaw in this argument is obvious - and neatly serves to indicate how the "planet" label isn't really that significant from any technical point of view.

  • Re:If it's round (Score:3, Informative)

    by mdwh2 ( 535323 ) on Thursday June 14, 2007 @07:17PM (#19513465) Journal
    At the least, Ceres, Vesta, Pallas and Hygiea ( &feedId=online-news_rss20F53 [] ). Admittedly not as many as I'd first thought there might be.
  • Re:If it's round (Score:3, Informative)

    by jonatha ( 204526 ) on Thursday June 14, 2007 @07:50PM (#19513687)
    Ceres and Vesta
  • by CorSci81 ( 1007499 ) on Thursday June 14, 2007 @08:03PM (#19513801) Journal

    Pluto won't clear its orbital path in the lifetime of the sun for a few reasons. The Kuiper Belt (where Pluto resides) is a very excited region of the solar system in terms of orbital eccentricities and inclications, which results in a very high mean impact velocity between objects out there. This means impacts tend to be destructive rather than letting anything grow larger. Additionally the number of particles in the Kuiper belt is sufficiently small impacts are fairly rare. Basically, the Kuiper Belt never became a planet for much the same reason the asteroid belt hasn't.

    Regarding the two planets bit, that's a highly unstable orbital configuration. If they orbited a common center of mass (like the Earth and moon) it would be feasible, but then we'd just call them a twin planetary system, or the smaller of the two would be considered a moon.

  • Re:It was a nickname (Score:4, Informative)

    by The One and Only ( 691315 ) <[ten.hclewlihp] [ta] [lihp]> on Thursday June 14, 2007 @09:32PM (#19514415) Homepage
    However, Eris's moon Dysnomia was named after a mythological demon of lawlessness.
  • by bdeclerc ( 129522 ) on Friday June 15, 2007 @03:34AM (#19516249) Homepage
    Actually, while the measurement of the extrasolar planets with "known" masses is likely to be considerably less precise than the mass measurements of Eris & Pluto, it is by no means as "based on models" as you imply.

    Those planets for which actual density has been determined are in a special class (or at least, special from our viewpoint):
    These are planets which (a) pass in front of their star as seen from earth, thereby causing a slight dimming of the starlight seen from here, and (b) have sufficient mass to cause a measurable red/blueshift in the spectrum of their parent star.

    The dimming of the light gives us their apparent diameter relative to their parent star, the duration of the dimming gives us a pretty accurate idea of the diameter of the star, the red/blueshift gives us their mass relative to their parent star and the orbital period gives us, to a considerable degree of accuracy, the mass of their parent star.

    These four parameters are actual measurements, so since we can derive the actual mass & diameter of the planet from these four parameters quite easily, the average density value we derive is as close to a direct measurement as we'll get.

    For planets which do not eclips their star as seen from earth, only lower limits to their mass can be determined (so the planet has to be "at least x earth masses") and even those do indeed depend on stellar modelling to determine the mass of the star, but since without the eclipses, there is currently no way to determine the diameter of said planet, there is no realistic way to determine the actual density of the planet anyway.

Information is the inverse of entropy.